The present invention relates to a scintillation crystal module used in a scintillation camera. More specifically, the invention relates to such a module which keeps its structural integrity even at elevated or depressed temperatures. In its most immediate sense, the invention relates to a method of retrofitting a purchased module so as to improve its structural integrity at high and low temperatures.
A conventional gamma camera includes a detector which converts gamma rays emitted from a patient into electrical signals. The detector includes a scintillation crystal and photodetectors which are in optical communication with the scintillation crystal. The gamma rays are directed to the scintillation crystal which absorbs the radiation and produces, in response, minute flashes of light. An array of photodetectors, which are placed in optical communication with the scintillation crystal, converts these flashes to electrical signals which are subsequently processed to form an image of that region of interest of the patient from which the radiation was emitted.
The scintillation crystal is usually a thallium-doped crystal of sodium iodide. The crystal deteriorates if exposed to moisture. Therefore, such a crystal is accommodated in a sealed module, which is usually purchased as a unit from a vendor. The module includes not only the crystal, but also an aluminum casing closed off by a glass window (usually with glass of the type marketed under the PYREX trademark). The window is provided so the photodetectors can respond to the scintillation events which take place within the crystal.
Conventional scintillation crystal modules include an internal seal which fills the gap between the glass window and the aluminum casing which surrounds it. However, when such modules are heated substantially above or cooled substantially below (as during shipment) room temperature, the seal frequently separates from the window and/or casing and the module admits moisture from the outside, causing the crystal to become unusable. This separation occurs because the aluminum casing and the glass window have significantly different thermal expansion coefficients and the forces which are generated during temperature changes cause the seal to break free of the parts to which it is attached.
Because of the great expense associated with replacement of the module, great attention has been paid to improving the internal seal within the module. However, the problem has remained unsolved and a seal which can maintain integrity of the module at high and low temperatures has not yet been developed.
It is thus a general object of the present invention to provide a scintillation crystal module which maintains the crystal in a sealed environment even at elevated and depressed temperatures.
Another object is, in general, to improve upon known scintillation crystal modules.
In accordance with the invention, a scintillation crystal module for use in a scintillation camera includes an external seal formed of moisture-impervious material which is attached to the outside of the module and bridges across the seal-filled gap between the casing and the window.
Preferably, the external seal is an epoxy polyamid or an elastomer such as a blocked urethane epoxy copolymer. Advantageously, the epoxy or epoxy copolymer is cast in a mold after the respective surfaces of the casing and the window have been ablated by sandblasting. The adhesive is introduced into the mold cavity and allowed to cure. A release agent is preferably applied to the mold surface to facilitate removal of the finished product from the mold.
By locating the external seal on the outside of the module and causing it to bridge the seal-filled gap between the window and the casing, the seal occupies a greatly increased volume and covers a greatly increased surface area. As a result, forces generated by the differing thermal expansion coefficients of aluminum and glass are distributed over a wider area and an increased volume and the seal is not broken when the module is subjected to high and low temperatures. As a result, the integrity of the module is maintained even at high and low temperatures.